Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental cash flow information
Interest paid	$ 6,375	$ 5,096	$ 4,329
Income taxes paid, net	11,514	10,005	10,578
Noncash transactions
Debt issued for purchases of businesses	(2,402)	(3,099)	(875)
Detail of acquisitions:
Accounts receivable	1,311	1,753	0
Operating supplies	23	0	157
Prepaid expense	89	128	55
Equipment	4,079	1,904	2,485
Other assets	7	52	0
Intangible assets	4,895	4,296	1,374
Goodwill	2,840	5,131	817
Liabilities assumed	2,381	2,713	216
Cash paid	$ 8,461	$ 7,452	$ 3,797